We provide novel evidence on the broader impacts of school choice systems by quantifying disparities in peer continuity from middle to high school in New York City. We find that Black and Hispanic students and those in high-poverty neighborhoods attend high school with a much smaller fraction of their middle school or neighborhood peers than their White, Asian, and low-poverty neighborhood counterparts. Disparities also emerge in peer isolation: 27% and 20% of Black and Hispanic students transitioned with no other student from their middle school, while only 7% to 8% of White and Asian students did. Group differences in choice similarity, which in part reflects systematic variation in the number of local school options, drive this result.